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Development of a distributed cogeneration combustor for the fuel straw

: Heidecke, Patric

Volltext (PDF; )

Österreichischer Biomasse-Verband, Wien:
4. Mitteleuropäische Biomassekonferenz 2014. Tagungsband : 4. Central European Biomass Conference 2014. Proceedings; 15. bis 18. Jänner 2014, Graz, Österreich
Heidenreichstein: Druckerei Janetschek, 2014
Mitteleuropäische Biomassekonferenz <4, 2014, Graz>
Central European Biomass Conference <4, 2014, Graz>
Abstract, Elektronische Publikation
Fraunhofer IFF ()

Supplying energy from renewable resources is becoming an increasingly important means to avert climate change. According to estimates, the energy recovered from biomass alone could sustainably cover 8 to 10 % of Germany’s primary energy consumption. While approximately 55 % of woody fuels’ potential is being exploited throughout Germany, the total available quantity of straw of 100 to 130 PJ/year remains virtually unused for heat recovery. In addition to its substantial, unutilized potential, grassy biomass has an additional advantage of rapid growth rates. Typically, an annual yield of 0.8 t/ha can be expected from forest wood waste. Fast growing woody plants (12 t/ha), cereal straw (5 t/ha) and miscanthus (18 t/ha) deliver higher annual yields. Although the recovery of energy from straw will grow in importance in the future, technical advances are tentative at present because of the significant differences between the properties of straw (low energy density and high alkali and chlorine content) and of woody fuels.
A fluidized bed combustor with a power range of a distributed plant (1-10 MW) was the technology used the joint project to recover heat from straw. The Fraunhofer IFF was in charge of researching the optimal operating conditions for straw combustors and identifying process parameters. Based on the findings, a fluidized bed combustor for cogeneration in the thermal power range of 1-4 MW was planned and engineered in detail. The Deutsches Biomasseforschungszentrum researched the basic logistical and economic conditions and environmental impacts of the planned model plant. The foci of the research project were the characterization of the fuel for combustion and the development of an appropriate plant design, which served as the basis for planning a model plant for a model site and assessing its economic and environmental impacts.
The issue of emissions was researched amply in extensive experimental studies. The carbon monoxide and nitrogen oxide emission standards (Technical Instructions on Air Quality Control) were met by applying primary measures to reduce pollutant gases. The addition of fuel additives containing calcium or the use of calcium-based bed materials allowed trouble-free continuous trial operation without restrictive ash fusion. Computational flow simulations were employed to validate the results of the engineering and to verify the working principle of the combustor designed. With straw’s special requirements on a fuel metering unit in mind, a continuously delivering and reliably functioning feeding system was designed, which allows for the specific conditions of combustion while feeding equally at two points in the combustion chamber. The problem of ash fusion identified beforehand was counteracted by developing a robust ash removal system.
As a follow-up to this joint project, a pilot plant will be constructed based on the concept and the base of fuels for such combustors will be expanded to include other culmiferous biomass (rape straw, miscanthus etc.).